1. Field of the Invention
The present invention relates to a catalytic material for removing nitrogen oxides in exhaust gas emitted from, for example, various kinds of combustion engines, to an apparatus for treating nitrogen oxides using the material, and to a method of removing nitrogen oxides.
2. Description of the Related Art
With environmental pollution having been raising a problem in recent years, a method of decomposing and removing harmful substances such as nitrogen oxides in exhaust gas is in urgent need of development. Examples of catalytic methods practically used for purifying exhaust gas include a three-way-catalyst method, which simultaneously removes carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx), and a selective catalytic reduction method, in which ammonia is used as a reductant. The three-way-catalysts in common use include a heat-resistant carrier, such as cordierite coated with .gamma.-alumina, supporting noble metals such as Pd, P and Rh. For the ammonia selective reduction method, oxides of metal such as V, Ti, and W are commonly used as the catalyst.
However, a three-way-catalyst has a drawback that the aforementioned noble metals are poisoned by excess oxygen existing in exhaust gas. As a result, their activities are significantly deteriorated. Therefore, the effect of purifying exhaust gas is available only in a narrow range around the stoichiometric air fuel ratio. In this connection, ion-exchanged zeolites have been studied for use as a catalyst which works even under an excess-oxygen atmosphere, but so far such zeolites have failed to exhibit sufficient heat resistance and water resistance.
By contrast, the ammonia selective reduction method is practically used in stationary combustion equipment as found in thermal power plants, and is capable of removing nitrogen oxides even when excess oxygen is present. However, this method involves a safety problem when it is used in urban areas or mobile units, due to toxicity and danger of ammonia.